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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

MILES AND GRINNELL GLACIERS                           427
,It accords well with the known history of the northern portion of Miles Glacier bulb to infer that when the glacier advanced across the Copper River valley to the site of Abercrombie Rapids, it also expanded southward to or near the site of the present terminal moraine, but probably not beyond it, This was at least 70 years ago (that is, before 1840), and it may well have been still earlier. In the southern section the ice has retreated from this stand, though it is still maintained in the zone of thickest ablation moraine in the northern portion of Miles Glacier at Abercrombie Rapids. The 70 year old trees on the outwash gravel plain indicate a similar expansion at the same period in the southern area.
A more recent advance, to be correlated with the one which extended across the zone of thick ablation moraine north of the lake, extended out to the present terminal moraine south of the lake, destroying any older moraines and vegetation between the lake 'and the moraine. After the terminal moraine was built the glacier again receded and vegetation sprang up on the surface. This was over 20 years ago, that is before 1890. The 22 year old shrubs north of the lake show that it was before 1888, but whether it was before the visits of Abercrombie and Allen in 1884-85, is not certain. Hayes' map made in 1891 and the 19 year old shrubs within a quarter mile of Miles Glacier, indicate that the retreat from this maximum was rapid, though it is to be noted that the single 19 year old bush may have had its start on the ice and have maintained growth during the melting of the ice beneath the moraine in which it grew.
The lack of destruction of all the older vegetation on the outwash plain suggests that the ice stood at the terminal moraine a very short time. The streams from the ice did destroy the vegetation in the border of the outwash plain, where the small alluvial fans form a rather barren fringe in front of the terminal moraine. They also cut swaths through the forest, now represented by the abandoned southwest-trending stream courses, in the bottoms of which we saw no 70 year old trees. These channels are cut so deeply below the general level of the plain that they suggest streams less heavily laden than those which deposited the outwash gravels. The channels are now overgrown with dense alder thickets, but we did not determine the ages of any of the shrubs in them. The absence of great numbers of dead cottonwoods standing in the stream courses seems to be due to the depth to which the channels were cut and the consequent removal of the tree trunks and roots; but some wood lies partly buried in these channels.
Scattered through the undergrowth, in the interstream areas, where the 70 year old cottonwoods grow, are a very few dead trees, standing upright in place, and a great many recumbent logs, evidently left by floods at the time of the cutting of the channels. That all the trees were not killed is an evidence of the shortness of the period of stream invasion of this forest. The short distance which the ice front had tore treat before the water from its melting would be diverted through the deep lake basin and find an outlet into the present channel of Copper River, at or near the railway bridge, doubtless helps to explain the lack of further destruction of the vegetation on the outwash plain during this episode.
An exceedingly detailed map of the portion of the moraine and outwash plain near Miles Glacier Station was made by the railway engineers. It has a contour interval of 2 feet and shows numerous shallow kettles due to slumping of ice formerly buried beneath the outwash gravels at the edge of the moraine; and it also shows some of the abandoned stream channels leading southwestward toward the Copper River. A conspicuous